Astronomers stumbled upon an "upside-down planet" that offers a new method for studying binary star systems. The planet is the first confirmed "self-lensing" binary star system - one in which the mass of the closer star can be measured by how powerfully it magnifies light from its more distant companion star.

Our Sun stands alone, but 40 percent of stars like it are actually part of two-star or multi-star systems, in which the stars' gravitational forces make them dance around each other.

University of Washington (UW) student astronomer Ethan Kruse, working under Eric Agol, was responsible for the discovery, though it was by sheer happenstance.

Astronomers detect far-away planets when they pass in front of, or transit, their host stars. The subsequent dimming light provides optimal conditions for viewing distant objects. Kruse was looking for transits when he saw something he didn't expect.

"I found what essentially looked like an upside-down planet," he said in a news release. "What you normally expect is this dip in brightness, but what you see in this system is basically the exact opposite - it looks like an anti-transit."

The white-dwarf, a star about as large as Earth but 200,000 times more massive, is 2,600 light-years (a light-year is 5.88 trillion miles) away in the Lyra constellation. It caused the observed increase in light rather than the conventional dimming because it was bending and magnifying light from its neighbor 43 million miles away via gravitational lensing, like a magnifying lense.

"The cool thing, in this case," Agol said, "is that the lensing effect is so strong, we are able to use that to measure the mass of the closer, white dwarf star."

This new technique - described in the journal Science - for studying systems that were otherwise too remote to detect and examine correctly brings astronomers one step closer to learning about the age of the galaxy.